Photoelectronic limit gauge



April 25, 1950 R. M. WILMoTTE ETAL 2,505,316

PHoToELEcTRoNIc LIMIT GAUGE v Fned April 1o, 1947 s sheets-sheet 1 RAYMOND M. W/LMOTTE ROBERT EBNED/GT April 25, 195o R. M. wlLMoTTE ETAL PHOTOELECTRONIC LIMIT GAUGE Flled April l0, 1 947 3 Sheets-Sheet 5 RAYMOND M W/LMOTTE 32# RO'BRTE. BENED/CT Patented Apr. 25, 1950l UNITED STATES PATENT OFFICE .Raymond..M. Wilmotte, Tarrytown, and .Robert E. Benedict, New -YorkfN. Y.: said Benedict .assigner to `said Wilmotte This invention relates to an electronic micromfeterffor giving a.substantially-continuous and instantaneous measurement of an objectwhich may `be .stationary or in .rapid motionxand toindicate Whether vthe vquantity lies .within Ipredetermined limits.. This application is related tolapplication .S..l.\1.` 698,345 filed September .'20, 1946, byNorman Taylor, .now abandoned.

The invention .disclosed in vthis application is an improvement of the electronic micrometer de 4scrilcled -in, the above application. It is desirable .inemany productonprocesses to, obtain apermanent. `record of .the .measured ,quantity .and .Y itis one, object of our .invention to .accomplish ,this result.

.It is another object of our invention to record .the average .value insteadV of the .instantaneous value of the quantity.measuredv in order toprovide a clear presentation ofthe trend of theprolduction process, uncomplicated Vby unimportant rors dueto the use of afinite light source.

The invention will be more fnllyunderstood #from the -follow-ingy description "and drawing in which:

'lFig". l is a schematic illustration ofone embodi- `ment ofA our invention showing' various: novel fea .tures thereof,

Fig. 2 is a diagram showing the optical 'system utilized to avoid errors `dueftoth'e vfinite-light Fig. 3 lis a view showingajdevelopment of *the 4scanning drum, and

Fig. 4 is va yschematic viewshowing a modication of the apparatus .shown in'Figure k'1.

Referring Lto the drawing; there is shown a lamp I ,ya collimating lens 2, producing a beam of light falling on'another/.lens` 3.' A .photocell '4 is v'placedfso that the cathode .is atfthe-focal point `of-lens 3; :Likewise therlamp l'is'flat the .lfocal point of the lens 2. Betweenthe llenses'ffzand 3 is an apertured screen 1. The obect '.8 to be measured ,isy placedso as to= be .framed 'byone of vtheapertures of the Vscreen "l.- A rotationgdrum .-vsurroundsthe lamp l. The. drumsiscircum- 40 jlimit aperture is closed byA an amount equalrto the 2 lfe'rentially slotted so that a beam of light -willbe .transmitted at all times through Vone and .only .one of the apertures of the screen The drum r9 will badescribed in more detail with reference to Figure 3.

.Between the. lens sand the cathode .5 -atrans- .parent diffuser Hl is placed. By virtue of vthe vvoltage source -Ila voltage is developed y'across an .impedance i6 corresponding to the amount of .light reaching the cathode 5. The voltage across the impedance i6 is amplified by an amplifier I8 `and impressed on the vertical plates of an oscilloscope le. The drum 9 is rotated by a motor 20 through a shaft 2l. The motor 29 is energized from an A. C. source which is also connected to .the horizontal plates of the oscilloscope to provide a horizontal sweep. The speed of themotor is chosen soV that aseparate sweep will be provided for each .beam of light produced by the scanning drum 9.. For example the motor may rotate at a speed of 1800 R. P. M. and therscanning drum may have .four 90 degree slots, as shown in Figure `3, While lthe A. C. source has a frequency of. C,v P. S.

The rotation ofthe drum 9 sequentially illuyminates the apertures of the screen 'l in the following order: rst, the upper test aperture, secondly, the middle aperture which may represent the upper. limit, thirdly, lthe upper aperture again, and lastly the lowest aperture which may represent 4the lower limit. The apertures can be adjusted in width by an suitable means, such as the `micrometer operated shutters shown in applicaf --tion SIN. 698,345 mentioned above.- In setting up that the three traces they produce cn the cathode ray tube coincide. Then the test aperture is opened a distance eoual to the size of the object. The lower limit aperture is opened and the upper minus and plus tolerances, respectively.

The rotation 'of the drum 9 then causes illumination of the cathode 5 through the three apertures. 4The amount of light on the cathode 5 determines the height of the horizontal traces on 'tense as'the limit traces and can be further idenl'tied by kinking it by means of a notch ln'the lslots lil Vand l! of the drum 9.

When the size of the object deviates, the test trace will approach or passbeyond the limit traces. yWe have oper -r`atedthe oscilloscope so that a one inch deflection on the cathode ray tube represents 0.001 inch in the dimension of the object being measured, so that deviations of 0.0001 are readily discernible. The measured object may move during measurement without causing errors, provided its shadow falls within the test aperture.

In order to avoid errors the same light source, photocell, amplifier and indicator must be used to measure the light through all three apertures. This is obviously the case in our apparatus. Any variation in any component of our apparatus will aiiect all three traces in the same manner, and not change their relative positions. rl`his is an extremely important feature of our apparatus.

A diffuser I is used to avoid burning a spot on the photocell, that is illuminating one spot so intensely that it has a short life. practice to place a diffuser in front of a photocell and focus the light on the diiiuser. We have made the discovery that to avoid errors the light from the several apertures must be focused on a point on the cathode 5 of the photocell through the diiuser. We believe the explanation of this phenomenon is, iirst, that substantially the same area of the photocell must be used for the light from all three apertures because not all areas of the photocell are equally sensitive at all times vduring its life. Secondly the diffuser diiiuses each ray into a cone of light whose axis is an extension of the ray of light. Thus each ray of light, from whatever angle it arrives, forms an area of light around the focal point on the cathode of the photocell. If the light is focused on the diffuser, as customary, rays arriving at different angles will be diiiused over diierent areas of the photocell cathode. We have found that it is essential to the obtainment of high accuracy to depart from the usual practice and place the diffuser and photocell as described.

Referring to Fig. 2 a finite light source is shown at 34. Through the lens 2 the lowest point of the light source will produce a beam parallel to the ray from said point through the center 32 of the lens 2. The lowermost ray of this beam 'transmitted through the lens 2 is indicated at '36. Similarly the ray 35 is the uppermost ray of the beamAtransmitted through the lens 2 from the highest point of the light source 34. Itcan be seen that if the light source 34 has a finite extent it will produce diverging beams even 'though it is placed at the focus of the lens 2. Consequently an object placed outside of the rays 35 and 3e will not intercept as much light asl it would if placed between the rays 35 and 35. Hence in any system in which the size of the object is determined by the amount of light it interceptsan error would be produced if the object moved past the ray 35 or 36. In order to avoid such errors we have confined all the apertures of the screen 'I between the rays 35 and 3S. The image 33 of the light source 34 is focused on the cathode 5 by the lens 3. It will be evident that as long as the shadow of the ,object 3 tobe measured remains within the confines of the test aperture the amount of light blocked from the cathode 5 remains invariant when the object 3 moves along or across the light Lbeam a small distance, even though the entire ,tained if the lens 3 is large enough to intercept all. rays passing through the several apertures ofthe screen.

Figure 3 shows the side of the scanning drum @laid out as a flat sheet. The drum has four It is the 4 slots I0, I I, I2 and I3. These slots are each 90 degrees long and the slots I0 and II may be notched to produce an identification mark on the oscilloscope test traces. They are cut so that slot I0 will permit the uppermost test aperture of the screen 7 to be illuminated and then the middle aperture,V without interruption or overlapping. After this the test aperture will again be illuminated through the slot I I, followed by the illumination of the lowest aperture of the screen I through the slot I3. The slots of the drum may have vertical ends, with the end of one slot lying on the same vertical line as the beginning of the next slot. Alternatively the ends of the slots may be inclined to the vertical with the center of the inclined ends of consecutive slots lying on the same vertical line. This construction. is shown at the end of slot II and the beginning of slot I3.

Figure 4 shows a modification of the system disclosed in Fig. l which provides a permanent record of the average value of the measured quantitymwith respect to the predetermined limits. The optical system in Fig. 4 is the same as that in Fig. l except for the scanning drum. The scanning drum 3| of Fig. 4 could be the same as the drum 9 of Fig. 1 but we prefer to use a drum which will scan the object to be measured for about 300 degrees of the drum and scan each ofthe limit apertures for about 30 degrees of the drum. These diiierent scanning periods enable the testrecordings to be distinguished from the limit recordings. Also in this modiiication we rotate the drum at a much lower speed, for example, less than one cycle per second. ,The response of the photocell 4 is again developed across the impedance I6 and passed through a D. C. amplifier 28. The output of the amplifier 28 is averaged by an integrating circuit or device 29 and impressed on a recorder 30. The recorder is an ink recorder of the type manufactured by the Esterline Angus Company. In this recorder the pen is actuated by the impressed signal and the chart is driven at a constant speed by a motor within the recorder. There is of course no necessity for synchronizing the recorder with the rotation of the drum 3|. The indications on the chart will appear as shown inFig. 4. This system produces a permanent record of the magnitude of the object measured averaged over a substantial portion of the period during which each aperture of screen I is scanned. Further, the averaging circuit or device 29 absorbs electrical transients and rapid fluctuations of the signal which would complicate the record. As a consequence, a clean record .of the trend of the magnitude of the object in relation to the desired upper and lower limits is obtained.

The discoveries we have made and incorporated in the embodiments of our invention disclosed herein permit great accuracy, stability, and readability of-rapid measurements under production line conditions. Many variations of our invention will be apparent to any one skilled in this art. For example, the voltage source II in Fig. 4 may be alternating instead of direct and have an audio frequency and the output of theampliiier rectified before being applied to the integrating circuit. The scope of our invention is therefore not limited except by the prior art and the following claims.

`We claim:

1. Apparatus for comparing the size of an object with predetermined upper and lower limits of'said size, comprising a lamp, a phctocell, means for focusing the light from the lamp on the cathode of the photocell, an opaque screen interposed between the lamp and the photocell, an aperture in said screen Whose effective area is determined by the size of the object and two additional apertures corresponding to the said upper and lower limits of the size of the object respectively, a scanning drum surrounding the lamp and having staggered circumferential slots, means for rotating the drum so that the three apertures of the opaque screen are successively illuminated through the slots of the drum, a transparent diffuser in the path of the light and near the photocell, and means for continuously indicating the amount of light transmitted through each aperture.

2. Apparatus for comparing the size of an object with predetermined upper and lower limits of said size, comprising a lamp, a photocell, a transparent diffuser adjacent the photocell, means for generating three successive light beams and focusing said beams on the cathode of the photocell through the diffuser, means for causing the area of one of said light beams to correspond to the size of the object and the areas of the other two light beams to correspond to the upper and lower limits of the size of the object, and means for indicating the response of the photocell to eachrof the light beams.

3. The apparatus defined in claim 2 includingr means for distinctively marking the indications of the size of object.

4. In a photoelectric system, a, collimating lens, a iinite light source at the focus of the lens, a photocell, a second lens for concentrating the light from the collirnating lens on the photocell, a screen having a plurality of apertures at different distances from the exis of the collimating lens but lying entirely within a region illuminated by every point of the light source, and the sec ond lens being large enough to intercept every ray of light from said source passing through said apertures, whereby the amount of light prevented from reaching the photocell by an object framed by one of said apertures is invariant with movement of the object across or along the beams of light, and means for diusing the light reaching said photocell from said second lens.

5. Apparatus for comparing the size of an object with a predetermined limit of said size, comprising a lamp, a photocell, a light diffuser means for generating two alternate light beams, and focusing said beams on the cathode of the photocell through said light diffuser, means for causing the area of one of the light beams to correspond to a magnitude of an object and the area of the other beam to correspond to a predeter-n mined limit of said magnitude, means for amplifying the responses of said photocell, and an indicator connected to said last named means.

6. The apparatus defined in claim 5 including means for distinguishing the recording of the magnitude of the object from the recording of the predetermined limit of the magnitude.

RAYMOND M. WILMOTTE. ROBERT E. BENEDICT.

REFERENCES CITED UNITED STATES PATENTS Name Date Desch Oct. 24, 1939 Number 

